The present invention relates generally to telecommunication methods and devices and in particular to PCM communications over telephone lines and the like.
The vast majority of Public System Telephone Network (PSTN) lines communicate information digitally using Pulse Code Modulation (PCM) as the main telecommunication protocol. PCM is the sampling of the analog signal (most typically voice signals) to produce pulse amplitude modulated signals in which the amplitude of each of the pulses is directly proportional to the original analog signal at the instant of the sample. Each of these amplitude samples is then quantized by measuring the amplitude and comparing the measured value against a scale of amplitude values in which each number in the scale represents an amplitude. The most widely used scale is from 0 to 255 in which each value in the scale is represented by an 8-bit code. Each amplitude sample is then transmitted as a binary coded signal representing the original amplitude of the signal at the sampling instant in time. These binary codes are then transmitted digitally over the telephone line and are decoded at the receiving end to reconstruct the original analog signal. Even if the sampling rate of the analog signal is more than twice the frequency of the analog signal, the reconstructed analog signal is not the exact duplicate of the original analog signal due to the quantization. In other words, if an analog amplitude falls between two values in the quantization scale, one of the values is chosen as the closest match. On the decoding end of the transmission, the exact amplitude value chosen on the amplitude scale is used for reconstructing the analog signal. The difference between the original amplitude and the reconstructed amplitude is called quantization error or quantization noise. There are two different PCM encoding and decoding protocols used in the world: xcexc-law (mu-law) coding (used the United States of America and other places) and A-law (used in Europe, Israel and other places). These two coding laws assign different quantization values for an analog signals. The two systems often have different quantization errors.
Modem communications over telephone lines have used a modulated analog signal which is modulated to encode digital data for digital communications between computers. The modulated analog signal of the modem is treated as any other analog signal by the PCM encoders and decoders. This form of modem communication was limited in speed due to the limited bandwidth allocated to voice telephone lines and due to quantization error or noise.
Recently, an improvement in the speed of modem communication has come in the form of PCM modems also known as 56K modems. These modems use the telephone line as a digital line. The International Telecommunications Union (ITU) recently issued a new recommendation for these modems designated Recommendation V.90. V.90 modems are designed for connections which are digital at one end and have only one digital-to-analog conversion in the path. That is, an information provider such as an Internet Service Provider (ISP) connects a digital modem directly to a PCM line at the telephone company while a user (such as in a home) connects a V.90 modem to an analog telephone line. Between the user and the telephone company, only a single analog to digital conversion is made.
From a V.90 modem user""s prospective, downstream (from the Telephone Company Central Office to the user) speeds of up to 56,000 bits per second (bit/s) are possible, depending on telephone line conditions, with upstream (from the user to the information provider) speeds of up to 33,600 bit/s. The modems that conform to the ITU V.90 protocol use ITU V.34 protocol on the upstream. The downstream transmission is done by PCM (Pulse Coded Modulation) such that downstream quantization noise is eliminated. However, downstream data transmission is impaired by other factors such as distortions and non-linearity in the telephone line, length of the line and equipment at the telephone company. Recently, great progress has been made in the development of methods for combating noise when transmitting in the downstream direction.
Problems of noise on the upstream pose a different set of problems. Quantization noise is now present due to the analog to digital conversion on the upstream. Also, non-linearity in the telephone line, length of the line, analog noise and equipment at the telephone company introduce noise into the upstream. In addition, the telephone companies themselves intro noise through Robbed Bit Signaling (RBS). RBS is a technique used by the telephone companies on T1 lines that use digital transmission. The technique uses the least significant bit of the PCM code word to control telephone signaling functions such a dial tone, ring, busy, answer, etc. This signaling method negatively effects V.90 by reducing the data rate on the v.34 connections on the upstream.
Thus, most upstream noise can be classified as one of two factors, generally described as PCM noise. The first factor is logarithmic quantization noise such as either xcexc-law or A-law quantization in which the quantization interval grows with the magnitude of the signal. The second factor is Robbed Bit Signaling (RBS) in which the least-significant bit of some of the samples may be overridden by control information that is transferred over the telephone network.
There are several methods proposed to combat the V.90 upstream or V.34 impairments. For example, the V.34 standard includes an option for using a non-uniform constellation that has a higher spacing around its high magnitude symbols, which are likely to suffer from PCM impairments more than the low magnitude symbols. Another class of approaches advocates error correction codes that are robust to a PCM noise, such as the 64 states trellis code proposed in the V.34 standard. However, practical experience shows that the effectiveness of these methods is limited. Thus, there is a need in the telecommunications art for combating PCM noise and improving data rate on the upstream of V.90 connections.
The present invention solves the above-mentioned problems in the art and other problems which will be understood by those skilled in the art upon reading and understanding the present specification. The present invention provides a method and device for combating logarithmic quantization and Robbed Bit Signaling (RBS) impairments that are typical to Pulse Code Modulation (PCM) telephone lines.
An apparatus is described which includes a front-end unit which receives samples of the digital PCM line, an impairment identifier unit which identifies samples that have a high likelihood to have large impairments due to the PCM line, an impairment estimator unit which estimates the value of impairment caused by the digital line, a samples reconstructor unit which fixes received samples by subtracting from them the value of the estimated impairment and an output unit which transfers the reconstructed samples to a receiver.
The present invention is particularly useful as a front end to a digital communications receiver that receives samples of the digital PCM lines. For example, but not by limitation, the present invention is useful as a front end to either V.34 receiver or V.90 receiver which are digitally linked to a T1 line. The present invention is further applicable to any receiver that receives PCM data from a digital communication line.
The present invention includes a method for combating PCM line impairments. The present invention allows improving signal quality at the output of the PCM line, and thus improving data rates and robustness of digital communication receivers, and particularly of V.34 receivers, that are digitally linked to the PCM line. This method can be implemented in a manner that requires small computational resources and can be added on top of all the methods described above for combating PCM impairments.